Dianne MarshEmerging Technology for the Enterprise

Conferencedmarsh@srtsolutions.com

03/26/2009

Practical Scala

Outline for Discussion

• Motivation and migration

• Demonstrate key implementation details

• Compare/contrast with Java

• Production Ready?

Quick overview

• Object-oriented/functional hybrid

• Download at www.scala-lang.org

• Runs on JVM

• Lightweight feel

• Static typing

Java 7 and Scala

• Java 7 (Likely)• Type Inference

• Closures

• Properties

• Operator Overloading

2010

• Scala• Type Inference

• Closures

• Properties

• Operators really functions

Now

Getting started with Scala

• No statics

• No operators (just methods)

• No primitives (just objects)

• if/else, while

• for (variants of)

• Immutables vs. mutables

Functional

(From David Pollak’s Roman Numeral Blog post: http://scala-blogs.org/2008/01/roman-numerals-in-scala.html)

Suspend for a while …

• Variants of for expressions

• Match expressions

• Traits vs. classes

Simple application (Swing)

• Used to scan rare books (online access, on-demand printing)

• Need to tag images with page numbers, page descriptions (TOC, etc.)

Why?

• Typical desktop app

• Simple functional component (numbering)

• Scala features in context

• Scala wraps Swing library

• If it can simplify Swing …

Overview

Features implemented

• Menu bar

• File chooser

• Dialog box

• Display image for selected filename

• Select range in table, enumerate page numbers by Roman Numeral or 123

Really Simple GUI Application

/********************** JAVA *************************/public class SimpleJavaApp {

public static void main(String []args) {SimpleJavaApp theApp = new

SimpleJavaApp();}SimpleJavaApp(){

JFrame frame = new JFrame("Title goes here");

frame.setSize(new Dimension(1200, 800));frame.setVisible(true);

}}

Really Simple GUI Application

/********************** JAVA *************************/public class SimpleJavaApp {

public static void main(String []args) {SimpleJavaApp theApp = new

SimpleJavaApp();}SimpleJavaApp() {

JFrame frame = new JFrame("Title goes here");

frame.setSize(new Dimension(1200, 800));frame.setVisible(true);

}}

/************************ SCALA ***********************/object SampleGUIApp extends

SimpleGUIApplication {def top = new MainFrame {

title = "First Scala Swing App"size = new java.awt.Dimension(1200, 800)

}}

Did you notice?

• No semicolons (optional)

• “object” specifies singleton

• “def” starts function definition

• title and size are properties

Next: add menu bar

/********************JAVA***************************/FileMenuBar menubar = new FileMenuBar(frame);frame.setJMenuBar(menubar);

Next: add menu bar

/********************JAVA***************************/FileMenuBar menubar = new FileMenuBar(frame);frame.setJMenuBar(menubar);

/******************* SCALA ****************************/def top = new MainFrame {

title = “Title goes here”menuBar = new MenuBar

}

Building the Menu

/***************** JAVA **********************************/public class FileMenuBar extends JMenuBar {

protected JMenu fileMenu;public FileMenuBar(ActionListener parent) {

fileMenu = new JMenu("File");JMenuItem item = new JMenuItem("Open...");item.addActionListener(parent);fileMenu.add(item);

item = new JMenuItem("Exit");item.addActionListener(parent);fileMenu.add(item);

// add all of the menus to the menubaradd(fileMenu);

}

Building the Menu

/***************** JAVA **********************************/public class FileMenuBar extends JMenuBar {

protected JMenu fileMenu;public FileMenuBar(ActionListener parent) {

fileMenu = new JMenu("File");JMenuItem item = new JMenuItem("Open...");item.addActionListener(parent);fileMenu.add(item);

item = new JMenuItem("Exit");item.addActionListener(parent);fileMenu.add(item);

// add all of the menus to the menubaradd(fileMenu);

}

/*************************** SCALA ***********************************/ val filemenu = new Menu("File")

filemenu.contents += new MenuItem(Action("Open"){// open file dialog

})filemenu.contents += new MenuItem(Action("Exit"){

dispose()})

menuBar.contents += filemenu

Handling events in Java

public class SecondAppFrame extends JFrame implements ActionListener {

public void actionPerformed(ActionEvent event){

if (event.getActionCommand().equals("Open...")) {}else if (event.getActionCommand().equals("Exit")) {}

}}

(We did this in Scala already)

Progress …

Menu bar

• Table• Class Parameters

• val vs. var

• Traits vs. Classes

• Pattern Matching

• Properties

Table

/************************ JAVA ************************/public class TableWithMenu extends JTable

implements ActionListener {TableWithMenu(ImagePageTableModel tm,

ActionListener parent) {super(tm);

}}

Table

/************************ JAVA**************************/public class TableWithMenu extends JTable

implements ActionListener {TableWithMenu(ImagePageTableModel tm,

ActionListener parent) {super(tm);

}}

/********************** SCALA *************************/var tableData: Array[Array[Any]]

val table = new Table(rows, cols) {model = new ChangeableDataTableModel {

rowData = tableDatacolumnNames = List("Filename", "Page

Num", "Tag")}

Scala Table Model

class ChangeableDataTableModel(var rowData: Array[Array[Any]], val columnNames: Seq[String]) extends AbstractTableModel {

override def getColumnName(column: Int) = columnNames(column).toStringdef getRowCount() = rowData.lengthdef getColumnCount() = columnNames.lengthdef getValueAt(row: Int, col: Int): AnyRef = rowData(row)(col).asInstanceOf[AnyRef]override def isCellEditable(row: Int, column: Int) = trueoverride def setValueAt(value: Any, row: Int, col: Int) {

rowData(row)(col) = valuefireTableCellUpdated(row, col)

}…

Table Model Notables

• Comparable to Java (not shown)

• AbstractTableModel data unchangeable

• Scala: convenient getter/setter for data

Traits

• Sophisticated mixins (1 or more)

• For reuse in multiple unrelated classes

• Can define behavior/maintain state

• Understand linearization if mixing in several (“super” dynamically bound)

Traits example

What Can’t Traits Do?

• No “class” params

• Use pre-initialized fields

Defining Traits (vs. Classes)

Trait:trait ChangeableDataTableModel extends AbstractTableModel{

var rowData: Array[Array[Any]] var columnNames : Seq[String]

…

Class:class ChangeableDataTableModel1(var rowData: Array[Array[Any]], var columnNames: Seq[String]) extends

AbstractTableModel

ParameterizingTraits (vs. Classes)

Trait:

val table = new Table(rows, cols) {model = new ChangeableDataTableModel {rowData = tableDatacolumnNames = List("Filename", "Page Num", "Tag")

}

Class:model = new ChangeableDataTableModel1 (tableData, List("Filename", "Page Num", "Tag"))

Properties in Scala

• Every var that is a non-private member of some object automatically gets a getter and a setter

• Can define getter/setter explicitly

• No special syntax

Properties Example

Properties (getters, setters) for vars in classvar rowData: Array[Array[Any]]

Generates (implementations for):def rowData : Array[Array[Any]]def rowData_=(d: Array[Array[Any]])

Accessed by caller as:tableModel.rowData = tableData

Java Listeners, Handlers

public class ImageSplitPane extends JFrame implements ActionListener, ListSelectionListener { . . .

table.getSelectionModel().addListSelectionListener(this);

public synchronized void valueChanged(ListSelectionEvent e) {

. . .}

}

Java Listeners, Handlers, cont.

// ImageSplitPane, cont …. . .

public void actionPerformed(ActionEvent event){

if (event.getActionCommand().equals("Exit")) handleExit(); else if (event.getActionCommand().equals("Open ...")) handleOpenFile();else if (event.getActionCommand().equals(“Roman”)) enumerateRoman();

. . .}

}

Scala Listeners, Handlers

val editUI = new BoxPanel(Orientation.Vertical) {val numberBox = new ComboBox(List("123", "Roman", "Clear"))contents += numberBoxlistenTo(numberBox.selection)listenTo(table.selection)reactions += {

case TableRowsSelected(_, range, false) =>replaceImage(imagename)

case SelectionChanged(`numberBox`) =>processSelection(table, numberBox.selection.item)

case _ =>true

}. . .

Simple Pattern Matching

def processSelection(table: Table, selectedItem: Any) {selectedItem match {case "123" =>

val entered = startVal.toIntval startRow = table.selection.rows.toArray(0)table.selection.rows.foreach( r => table.update(r, 1, r - startRow + entered))true

case "Roman" =>functionalRomanEnumeration(table, startVal)true

case _ =>false

}

Match Expressions

• No fall-throughs between cases

• Must match something (use _)

• No “return” needed at end of block

• Always result in a value

More than just switch

override def rendererComponent(isSelected: Boolean, hasFocus: Boolean, row: Int, column: Int): Component =

ImageViewer.rowData(row)(column) match {case s: String => l.componentFor(this, isSelected, hasFocus, s, row, column)case _ => super.rendererComponent(isSelected, hasFocus, row, column)

}

Other Notables …

• Generics

• Interoperability

• Testing

• Production environment?

Generics

• Like Java, still uses erasure (type info not preserved at runtime)

• Supports variance annotations

Simple Generics Example

/************* JAVA ******************class Pair<K,V> {K key;V value;Pair(K key, V value) {

this.key = keythis.value = value

}

/*********************** SCALA ************************/class Pair[K, V] (val key: K, val value: V) {}

Java/Scala interoperability

• Scala Swing library code

• Daniel Spiewak’s blog: http://www.codecommit.com/blog/java

What about testing?

• JUnit 3 and 4 supported

• Scalatest

Using JUnit Tests in Scala

class TestRoman extends TestCase {def testRomanize = {

assertTrue(RomanNumeralConverter.romanize(5) == "V")assertTrue(RomanNumeralConverter.romanize(100) == "C")assertTrue(RomanNumeralConverter.romanize(7) == "VII")assertTrue(RomanNumeralConverter.romanize(1964) == "MCMLXIV")

}

def testValidFormat = {assertTrue(RomanNumeralConverter.isValidRoman("0") == false)

}

ScalaTest

• http://www.artima.com/scalatest/

• Supports behavior driven testing

• Traditional testing

• JUnit/ScalaCheck Integration

• TestNG style

Deployment?

• Can make a jar• Include scala-library.jar

• Include scala-swing.jar

• Size issues

• Scala bazaar

What Happened to FUNCTIONAL?

• Getting your code (brain?) ready• vals vs. vars

• Immutable collections

• Use pattern matching

• Start exploring map, filter, etc.

OK, just a taste …

Function literal

Placeholders for each parameter

Principles to like …

• Conciseness

• Uniform Access Principle

• Building Blocks + Libraries

Specifics to like …

• Type Inference

• Class Parameters

• Properties

• Pattern Matching

• For expressions

• Multiple return values using tuples

Ready … Set … Go?

• Still early adopter stage

• IDE: plugins for NetBeans, IntelliJ, Eclipse

• Documentation: sign of maturity

The JVM needs “Java 7”

• JVM improvements critical

• Tail call optimization

For more information …

• “Programming in Scala”, book by Odersky, Spoon, Venners on Artima

• “Beginning Scala” – David Pollak, alpha from APress

• “Programming Scala” (concurrency) – VenkatSubramaniam, May 2009, Prags

• Java Posse podcast (Roundup ’08 recording: “What does Scala Need”)

• Java Posse Roundup ’09 recordings …